ignore/types: add 'graphql'

[jparise]

GraphQL file extensions: .graphql and .graphqls (schema)

[jparise]

Whoops, I hadn't noticed #2439 was already proposing a graphql type. I think the file extensions I chose (.graphql, .graphqls) are slightly more correct, however:

• .graphql is the canonical GraphQL file extension (Recommend an extension to use graphql/graphql-spec#203)
• .graphqls is the recommended GraphQL SDL file extension (https://www.apollographql.com/docs/kotlin/essentials/file-types/)

I excluded .gql, a commonly used abbreviated extension. It could conflict with other existing types (e.g. https://filext.com/file-extension/GQL), although I doubt many people are going to use -tgraphql in contexts where non-GraphQL .gql files appear.

[BurntSushi]

I'll go with this PR for now, since it's more conservative.

We can add .gql later if there's demand. It's inevitable that some extensions will conflict. That's OK.